The present invention relates to a pump unit comprising a main pump and a charge pump having a variable pump capacity.
Publication DE 100 45 118 A1 makes known a hydraulic system comprising a main pump and a precompression pump. The hydraulic system presented therein includes two pumps in a serial arrangement, it being possible to adjust the pump capacity of each of the two pumps. The precompression pump, the delivery side of which is connected to an inlet line of the main pump, may be adjusted using an adjusting device. An open-loop or closed-loop control device which acts on a related adjusting mechanism of the precompression pump is provided for this purpose.
Publication GB 21 50 981 A also makes known a system which includes two pumps located in series. A charge pump suctions pressure medium out of a tank volume and delivers it via a connecting line to a main pump. Both the precompression pump and the main pump are designed to be variable, and each one includes a control device. An electronic control unit controls is provided to control the control device of the charge pump and the control device of the main pump.
In the known hydraulic systems that include two serially connected hydraulic pumps, it is disadvantageous that the pump capacity of the first pump, which supplies the second pump with pressure medium, is not regulated as a function of the pressure that acts on the inlet side of the second pump. Therefore, the control mechanisms only indirectly take into account the operating situation of the total system, which is composed of both pumps.
Furthermore, publication DE 103 53 027 A1 makes known a simple vane pump, in the case of which a pressure differential between a delivery pressure and an intake pressure acts on a stroke ring. The stroke ring may be displaced in a translational manner, and it is acted upon by the delivery-side pressure on one side, and, on the other side, by the pressure that is present on the intake side of the vane pump.